Neato Botvac D85 Battery temperature misreading

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Neato Botvac D85 Battery temperature misreading

Postby GijsbertvL » March 29th, 2018, 8:33 am

Hi All,

My Botvac D85 has a strange failure. It says, Please wait while I cool down my battery. (not so strange right?)But..
The battery isn't warm at all. I changed it for a new one, but even before I can recalibrate for new battery I get this message right away.

So I used Neato control to see what temp it reads: the temp jumps around from 35 degrees Celsius to 75 degrees Celsius and back and forward within a second.

Does anyone know how to fix this (or even shut down the sensor if possible)?

Many thanks for your reply!

Gijsbert
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Re: Neato Botvac D85 Battery temperature misreading

Postby glnc222 » March 29th, 2018, 8:50 pm

I examined this circuit in detail only on the XV models but the same principles apply and the components might be the same. Temperature is read by the cpu as a voltage on an analog input. The voltage is from the junction of about a 20K ohm resistor on the system board connected to the 5v supply, and the passive thermistor, on the yellow wires, in the NiMh battery pack -- a voltage divider on 5V. The thermistor is specified as 10K ohms at 25 C temperature and and resistance falls as the temperature rises, lowering the voltage sensed. So a disconnected thermistor reads very cold -- and there is an error msg. for a missing thermistor (as the charging procedure uses temperature to detect the full charge state, with NiMh chemistry batteries). There are then two ways the sensing can be altered: some type of shorting out the thermistor would read a very high temp, and some breakdown of the on board series resistor raising its resistance could do the same. Another possibililty is drop in the 5v power supply, some defect in the voltage regulator circuit, giving a lower voltage on the thermistors. Other sensors run on 5v so could also be affected in this case. Observing the 5v supply on say, the Wall or Cliff sensor (red wire) while the temperature varies would reveal whether the power supply is at fault (you could also locate on the system board the end of the series resistor connected to 5v -- and some breakdown in that connection could also be a source of wrong readings).
An ammeter inserted between the thermistor and the system -- at the battery connector -- would show whether the on board resistor is not stable, by showing whether current diminishes from a higher than normal resistance there (normally as the thermistor heats up and falls in resistance, current would increase because of less total resistance from the entire voltage divider).
Examining voltage at the thermistor, on the battery connector, would reveal whether the voltage actually changes with the strange temp readings, or whether the cpu is just not reading it correctly.

[edit] A poor battery connector connection, creating a resistance in the thermistor wires, would also work the same as an excessive on-board series resistance. One wire goes to system ground, and the other to the series resistor. A bad ground connection would read cold -- raising the junction voltage from an apparent high thermistor resistance. A bad drive connection would read high temp.

Observing voltages at the system board end of the battery connector cable, and at the battery, would detect whether the connector is bad creating a voltage difference across the connector.
The connector can be replaced. On the XV models with a different, Molex connector sometimes the large hollow pins get loose and need to be tightened, but the Botvac has a better connector which ought not have that problem.
glnc222
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Re: Neato Botvac D85 Battery temperature misreading

Postby GijsbertvL » March 30th, 2018, 7:07 am

Hi Glnc,

Many thanks for your swift and extensive reply. I will examen the main board based on what you have written. I will keep you posted with the results.

Thanks again!
GijsbertvL
 
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Re: Neato Botvac D85 Battery temperature misreading

Postby valamilamasag » March 30th, 2018, 9:01 am

glnc222 wrote:...

Totally wrong answer. The botvac uses the fuel gauge IC to measure the temp not an analog input on the main SOC/application processor. You can't measure anything without an oscilloscope to determine if its working or not.
The 5V rail of the fuel gauge IC is a different 5V rail than the rest of the circuit. Also the pull up is not always on.

GijsbertvL wrote:...

Since you replaced the battery its probably a fault on the motherboard.

Disconnect the battery and check ThermistorPresent returned by neato control (or by command line with "getcharger"). It should be 0.
With a connected battery it will be "1". Next check "BatteryTemperature" (not BattTempCAvg). If that jumps around then you are in no luck.
You have the same failure that I had (analog input of the fuel gauge IC gone bad).

You can try to replace the filter capacitor, the pull up resistor (both 0402 size parts) and the fuel gauge IC itself. This needs good soldering skills/equipment. The 0402 resistors are 0.4mm by 0.2mm (as the name suggests).
If you can do this I can provide you with additional info / images what to check/replace.
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Re: Neato Botvac D85 Battery temperature misreading

Postby glnc222 » March 30th, 2018, 6:53 pm

Yes I forgot the Botvac has the Maxim fuel guage IC. This IC has been replaced by users in some units with incorrect charging behavior. Besides temperature, the main function is measuring the charge into and out of the battery (current x time),off loading that work from the main cpu. Charge usage is monitored to preserve a margin for returning to base, as well as a buffer against damaging deep discharge of the battery.
Replacement requires surface mount soldering techniques and tools, the hot air method. Links in the Mods & Repairs list top of forum.
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